The present invention relates to magnetic resonance imaging apparatus and procedures. In magnetic resonance imaging, an object to be imaged as, for example, a body of a human subject is exposed to a strong, substantially constant static magnetic field. The static magnetic field causes the spin vectors of certain atomic nuclei within the body to randomly rotate or “precess” around an axis parallel to the direction of the static magnetic field. Radio frequency excitation energy is applied to the body, and this energy causes the precessing atomic nuclei to rotate or “precess” in phase and in an excited state. As the precessing atomic nuclei relax, weak radio frequency signals are emitted; such radio frequency signals are referred to herein as magnetic resonance signals.
Different tissues produce different signal characteristics. Furthermore, relaxation times are the dominant factor in determining signal strength. In addition, tissues having a high density of certain nuclei will produce stronger signals than tissues having a low density of such nuclei. Relatively small gradients in the magnetic field are superimposed on the static magnetic field at various times during the process so that magnetic resonance signals from different portions of the patient's body differ in phase and/or frequency. If the process is repeated numerous times using different combinations of gradients, the signals from the various repetitions together provide enough information to form a map of signal characteristics versus location within the body. Such a map can be reconstructed by conventional techniques well known in the magnetic resonance imaging art, and can be displayed as a pictorial image of the tissues as known in the art.
The magnetic resonance imaging technique offers numerous advantages over other imaging techniques. MRI does not expose either the patient or medical personnel to X-rays and offers important safety advantages. Also, magnetic resonance imaging can obtain images of soft tissues and other features within the body which are not readily visualized using other imaging techniques. Accordingly, magnetic resonance imaging has been widely adopted in the medical and allied arts.
Several factors impose significant physical constraints in the positioning of patients and ancillary equipment in MRI imaging. Many MRI systems use solenoidal superconducting coils to provide a static magnetic field. The coils are arranged so that the patient is disposed within a small tube running through the center of the coils. The coil and tube typically extend along a horizontal axis, so that the long axis or head-to-toe axis of the patient's body must be in a horizontal position during the procedure. Moreover, equipment of this type provides a claustrophobic environment for the patient.
Iron core magnets have been built to provide a more open environment for the patient. These magnets typically have a ferromagnetic frame with a pair of ferromagnetic poles disposed one over the other along a vertical pole axis with a gap between them for receiving the patient. The frame includes ferromagnetic flux return members such as plates or columns which are located outside the patient-receiving area and extend vertically. A magnetic field is provided by permanent magnets or electromagnetic coils (superconductive or resistive) associated with the frame. A magnet of this type can be designed to provide a more open environment for the patient. However, it is still generally required for the patient to lie with his or her long axis horizontal.
Recently, ferromagnetic frame magnets having horizontal pole axes have been developed. As disclosed, for example, in commonly assigned U.S. Pat. Nos. 6,414,490 and 6,677,753, the disclosures of which are incorporated by reference herein, a magnet having poles spaced apart from one another along a horizontal axis provides a horizontally oriented magnetic field within a patient-receiving gap between the poles. Such a magnet can be used with a patient positioning device including elevation and tilt mechanisms to provide extraordinary versatility in patient positioning. For example, where the patient positioning device includes a bed or similar device for supporting the patient in a supine or recumbent position, the bed can be tilted and/or elevated so as to image the patient in essentially any position between a fully standing position and a fully recumbent position, and can be elevated so that essentially any portion of the patient's anatomy is disposed within the gap in an optimum position for imaging. As further disclosed in the aforesaid applications, the patient positioning device may include additional elements such as a platform projecting from the bed to support the patient when the bed is tilted upright for a standing or sitting orientation. Still other patient supporting devices can be used in place of a bed in a system of this type. Thus, magnets of this type provide extraordinary versatility in imaging.
FIG. 1 of the current application shows a sectional view of an MRI magnet subsystem 100. MRI magnet subsystem 100 includes a magnet having a ferromagnetic frame 102, a flux generating means as is described in further detail below, and a patient handling system 106. The ferromagnetic frame 102 includes a first side wall 108 and a second side wall. For purposes of clarity, FIG. 1 does not show the second side wall or any of its associated structures. The side walls extend vertically. The ferromagnetic frame 102 also includes a top flux return structure 112 and a bottom flux return structure 114. The top flux return structure 112 may include two columns 116 and 118. Between these two columns, a top opening 120 is defined. Similarly, the bottom flux return structure 114 may include two columns 122 and 124 that together define a bottom opening 126. Thus, the side walls and flux return structures 112 and 114 form a rectilinear structure, with the top flux return structure 112 constituting the top wall of the rectilinear structure, the bottom flux return structure 114 constituting the bottom wall of the rectilinear structure and the side walls forming the side walls of the rectilinear structure. The frame 102 of the rectilinear structure defines a front patient opening 128 on one side of the frame 102 and a similar back patient opening 130 on the opposite side of the frame 102. The ferromagnetic frame 102 further includes a first magnetic pole 132 and a second magnetic pole. The first magnetic pole 132 extends from the first side wall 108 towards the second side wall and the second magnetic pole extends from the second side wall towards the first side wall 108. The magnetic poles are generally cylindrical and are coaxial with one another on a common horizontal polar axis 136. Between the magnetic poles is a gap 131, also referred to as the patient-receiving space, of the magnet. The gap or patient-receiving space 131 is accessible by the front patient opening 128, the back patient opening 130, the top opening 120 or the bottom opening 126.
The flux generating means includes a first electromagnetic coil assembly 138 which surrounds the first magnetic pole 132, and a second electromagnet coil assembly, which surrounds the second magnetic pole in a like fashion. These electromagnetic coil assemblies may be either resistive or superconductive.
The patient handling system 106 is capable of three degrees or axes of motion. The patient handling system 106 may be termed an upright patient handling system, although the patient handling system 106 is not limited to standing position applications and may include sitting and other upright positions, as well as the recumbent position. The patient handling system 106 includes a carriage 142 mounted on rails 144. The carriage 142 may move linearly back and forth along the rails 144. The rails 144 typically do not block the bottom open space 126. A patient handling system operative in the manner described herein is disclosed in U.S. application Ser. No. 09/918,369, filed on Jul. 30, 2001, which is entitled “Positioning System For An MRI,” the disclosure of which is incorporated by reference herein.
A generally horizontal pivot axis 146 is mounted on carriage 142. An elevator frame 148 is mounted to the pivot axis 146. The carriage 142 is operable to rotate the elevator frame 148 about the pivot axis 146. A patient support 150 is mounted on the elevator frame 148. The patient support 150 may be moved linearly along the elevator frame 148 by an actuator 152. Thus, a patient 154 can be positioned with a total of three degrees of freedom, or along three axes of movement or motion. Specifically, the patient handling system 106 can move a patient 154 in two linear directions and also rotate the patient 154 around an axis. The arrows 155 of FIG. 1 show the three axes of movement possible with the patient handling system 106. Note that often the rails 108 are mounted such that portions of patient 154 may be positioned below the rails through bottom open space 126.
Often, a foot rest 158 may be used in order to support a patient in a standing position. Given the wide variety of positions possible with the patient handling system 108, many other such supports may be implemented, such as chair seats or straps.
The patient handling system 106 incorporates one or more actuators 152 and an actuation control unit 153. Actuators 152 may be conventional electrical, electromechanical, pneumatic, hydraulic or other devices capable of imparting the desired motion to the elements of the patient handling system. For example, the actuators may include elements such as conventional stepper motors or other conventional electric motors linked to the elements of the patient handling system 106. The actuator control unit 153 may incorporate a conventional programmable controller, microprocessor, or computer with appropriate input and output interfaces. As further discussed below, the actuation control unit 153 is linked to a control computer and to the manual controls which regulate the patient handling system. The actuation control unit may be mounted in proximity to the actuators 152 as, for example, on carriage 142.
The MRI magnet subsystem 100 with patient handling system can be contrasted with an older MRI system such as shown in FIG. 2A. The MRI apparatus 200 has a magnet canopy 202 and a bed 204 on which the patient 206 lies recumbent. The bed 204 is typically capable only of linear motion to the left and right in the orientation of FIG. 2B. This linear motion is restricted to a horizontal plane inside the magnet bore. Thus, many of the advantages of the patient handling system as discussed in the aforementioned applications are unavailable. A control panel 208 with simple controls 210 may be mounted directly to the magnetic canopy 202. Alternatively, the control panel 208 may be mounted directly to the bed 204.
In addition to apparatus for magnetic resonance imaging described above, U.S. Pat. No. 5,008,624 to Yoshida (“Yoshida”) discloses a magnet having a pair of super conductor blocks one facing each other placed at two ends of a metallic U-shaped frame. Yoshida's magnet further includes a patient carrier in the form of a chair equipped with a lifting mechanism and a reclining mechanism. Yoshida further discloses that by rotating the U-Shaped frame of the magnet or by lifting up and down the patient carrier with lifting mechanism, various relative orientations of the main magnet and the patient carrier are realizable.